The invention relates to the insulation of the rotor end windings of electrical machines.
Electrical machines such as generators or electric motors comprise a housing with a stator and a rotor and also in each case a stator winding and a rotor winding accommodated in corresponding slots. At each end of the rotor, the rotor windings form a rotor end winding. In the region of the rotor end windings, the rotors are provided with rotor end bells made of steel, which serve inter alia for fixing the axial end regions of the rotor windings to resist centrifugal forces. For assembly, the rotor end bells are placed axially onto the ends of the rotor body and shrink-fitted on there.
Such rotor end bells are subjected to great mechanical stress during the operation and overspeed testing of the electrical machine. In particular in the case of limit-rating machines, i.e. for machines in which the windings are of a considerable weight and the circumferential speeds are high, significantly greater loads are to be expected on account of the centrifugal force. However, high thermal and mechanical loads also already occur during assembly, mainly as a result of the shrink connection.
The rotor end bells are generally of a two-part form and comprise an axially arranged retaining ring and a radially directed retaining ring liner plate, as already known from DE 197 32 949 A1, which relates to a turbogenerator. In this case, the retaining ring and retaining ring liner plate may have both a screw connection and a shrink connection.
In the case of GB 820577, the insulating layers of the conductor bars of the rotor end winding are covered prior to the fitting of the rotor end bell by means of a non-magnetic retaining ring liner sheet serving for the mechanical protection of these insulating layers. For example, retaining ring liner sheets made of sheet steel or else sheet copper are used (K. Weigelt, xe2x80x9cKonstruktionsmerkmale grosser Turogeneratorenxe2x80x9d [design features of large turbogenerators] in ABB Technik 1/1989, page 13).
However, it has been found that, in particular for retaining ring liner sheets of limit-rating machines, the previously used materials are not suitable in the long term. Although steel has adequate strength, it has a high modulus of elasticity, i.e. high-strength alloys are necessary to allow appropriate adaptation to the expansion of the rotor end bell occurring during the operation of such a turbogenerator. One disadvantage of these high-strength steel alloys is the unfavorable pairing of materials in contact with the rotor end bell. By contrast, the elasticity of copper is adequate, but it is not strong enough to withstand the deformations of the rotor end bell occurring during the start-stop cycles. Aluminum, which has a very low modulus of elasticity and adequate strength, is not as well suited however for the temperatures in excess of 300xc2x0 C. required for the shrink-fitting of the rotor end bell in the case of limit-rating machines. Other previously used materials, such as plastics for example, likewise do not have the properties required for limit-rating machines, such as temperature stability and mechanical elasticity, either.
The invention attempts to avoid all these disadvantages. It is based on the object of providing improved protection for the insulation of the rotor end winding of electrical machines that is suitable in particular for limit-rating machines.
According to the invention, this is achieved by the rotor end bell consisting of steel and the retaining ring liner sheet consisting of titanium or a titanium alloy. In principle, all commercially available titanium alloys are suitable for the production of the retaining ring liner sheet, TiAl6V4 being used in particular for reasons of cost and availability. Both titanium and titanium alloys have high strength, a low modulus of elasticity and a high temperature stability, so that they are particularly well-suited for use as the retaining ring liner sheet.
It is particularly advantageous for a lubricant with a temperature stability of  greater than 300xc2x0 C. to be applied between the retaining ring liner sheet and the rotor end bell, with both the retaining ring liner sheet and the rotor end bell being suitable as a supporting material for the lubricant. The lubricant makes it easier for the rotor end bell to be pulled on and detached later, and also prevents sliding abrasion between the retaining ring liner sheet and the rotor end bell.
Finally, a lubricating varnish is advantageously applied as a lubricant, such as the molybdenum-containing lubricating varnish known by the trade name MOLYCOTE 3402, or a metal coating is used, such as for example plasma-sprayed copper or aluminum. In the latter case, a non-magnetic metal coating proves to be particularly advantageous, because this does not allow the abrasion products possibly occurring to be entrained into the magnetic field.
It is only by extensive friction-value tests for examining the behavior of the claimed pairings of materials that the suitability of the combination of a rotor end bell produced from steel with a retaining ring liner sheet made of titanium or a titanium alloy and the claimed lubricants has been demonstrated. Since they are, nevertheless, known materials and application methods, the costs involved in their use are relatively low.